A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Printed circuit boards and IC's are normally processed several times to create several patterns on top of each other. It may be very important that the target portions are located at precisely determined positions above each other; this is usually called overlay. Good overlay is obtained by aligning the substrate with respect to the patterning structure, which is usually called alignment.
Conventionally substrates are provided with alignment marks whose positions relative to the target portions are known. During alignment an alignment sensor measures the positions of the alignment marks. In this way, the positions of the target portions may be determined. The alignment sensor can only view a small area on the substrate at a given time (this can be considered to be the footprint of the alignment sensor). Often, when alignment is begun, the alignment mark does not coincide with the area viewed by the alignment sensor. To solve this problem, the substrate is scanned underneath the alignment sensor, over a distance sufficiently large that it is certain that the alignment mark passes through the area viewed by the alignment sensor. The position of the alignment mark is measured as it passes through the area viewed by the alignment sensor.
A possible disadvantage of this alignment method is that scanning of the substrate is time consuming, and thus effects the number of substrates that can be processed by the lithographic apparatus per hour.
Instead of using alignment marks, U.S. Pat. No. 3,898,617 describes alignment by measuring positions of circuit features, i.e. alignment is direct to the circuit features. The alignment sensor views a small area of a target portion on the substrate. The sensor records an image of the circuit features located in the area of the target portion. The image is compared to a library which comprises images of circuit features and their associated locations. When a match is found between the measured image and a library image, the associated location retrieved from the library gives the location of the substrate.
A possible disadvantage of this method is that a time consuming scan of the substrate underneath the alignment sensor may be needed until a recorded image is found which matches an image in the library.